Flexographic printing plate with improved storage stability

ABSTRACT

A method of making a relief image printing element comprising a plurality of relief printing dots. The method includes the steps of imagewise exposing at least one photocurable layer to actinic radiation to selectively crosslink and cure portions of the at least one photocurable layer; and developing the relief image printing element to separate and remove uncrosslinked and uncured portions of the at least one photocurable layer to reveal the relief image therein. The at least one photocurable layer includes a photoinitiator exhibiting a quantum yield of initiation (Qi) of more than 0.05 at a 365 nm wavelength. The substrate has an optical density from 0.5 to 5 in the wavelength range of 365 nm to 450 nm but preferably allows transmission of from 0.1% to 10% of incident light in the wavelength range of 365 nm to 450 nm.

FIELD OF THE INVENTION

The present invention relates generally to printing plate formulationsfor producing flexographic printing plates with improved storagestability.

BACKGROUND OF THE INVENTION

Flexography is a method of printing that is commonly used forhigh-volume runs. Flexography is employed for printing on a variety ofsubstrates such as paper, paperboard stock, corrugated board, films,foils and laminates. Newspapers and grocery bags are prominent examples.Coarse surfaces and stretch films can be economically printed only bymeans of flexography. Flexographic printing plates are relief plateswith image elements raised above open areas. Generally, the plate issomewhat soft, and flexible enough to wrap around a printing cylinder,and durable enough to print over a million copies. Such plates offer anumber of advantages to the printer, based chiefly on their durabilityand the ease with which they can be made.

A typical flexographic printing plate as delivered by its manufactureris a multilayered article made of, in order, a backing, or supportlayer; one or more unexposed photocurable layers; optionally aprotective layer or slip film; and often a protective cover sheet.

The support sheet or backing layer lends support to the plate. Preferredmaterials for the backing layer include sheets made from syntheticpolymeric materials such as polyesters, polystyrene, polyolefins,polyamides, and the like. Generally the most widely used support layeris a flexible film of polyethylene terephthalate. The support sheet canoptionally comprise an adhesive layer for more secure attachment to thephotocurable layer(s). Optionally, an antihalation layer may also beprovided between the support layer and the one or more photocurablelayers. The antihalation layer is used to minimize halation caused bythe scattering of UV light within the non-image areas of thephotocurable resin layer. For most flexographic applications the supportlayer is clear, particularly when the establishment a photopolymer floorin the printing plate is desired.

The photocurable layer(s) include photopolymers, monomers, initiators,reactive or non-reactive diluents, fillers, and dyes. The term“photocurable” refers to a composition which undergoes polymerization,cross-linking, or any other curing or hardening reaction in response toactinic radiation with the result that the unexposed portions of thematerial can be selectively separated and removed from the exposed(cured) portions to form a three-dimensional relief pattern of curedmaterial. Preferred photocurable materials include an elastomericcompound, an ethylenically unsaturated compound having at least oneterminal ethylene group, and a photoinitiator. Photocurable materialsare disclosed, for example, in European Patent Application Nos. 0 456336 A2 and 0 640 878 A1 to Goss, et al., British Patent No. 1,366,769,U.S. Pat. No. 5,223,375 to Berrier, et al., U.S. Pat. No. 3,867,153 toMacLahan, U.S. Pat. No. 4,264,705 to Allen, U.S. Pat. Nos. 4,323,636,4,323,637, 4,369,246, and 4,423,135 all to Chen, et al., U.S. Pat. No.3,265,765 to Holden, et al., U.S. Pat. No. 4,320,188 to Heinz, et al.,U.S. Pat. No. 4,427,759 to Gruetzmacher, et al., U.S. Pat. No. 4,622,088to Min, and U.S. Pat. No. 5,135,827 to Bohm, et al., the subject matterof each of which is herein incorporated by reference in its entirety.More than one photocurable layer may be used.

The photocurable materials generally cross-link (cure) and hardenthrough radical polymerization in at least some actinic wavelengthregion. The type of radiation used is dependent on the type ofphotoinitiator in the photopolymerizable layer. As used herein, actinicradiation is radiation capable of effecting a chemical change in anexposed moiety in the materials of the photocurable layer. Actinicradiation includes, for example, amplified (e.g., laser) andnon-amplified light, particularly in the UV and violet wavelengthregions. Any conventional sources of actinic radiation can be used forthis exposure step, including, for example, carbon arcs, mercury-vaporarcs, fluorescent lamps, electron flash units, electron beam units andphotographic flood lamps.

The slip film is a thin layer, which protects the photopolymer from dustand increases its ease of handling. In a conventional (“analog”) platemaking process, the slip film is transparent to UV light. In thisprocess, the printer peels the cover sheet off the printing plate blank,and places a negative on top of the slip film layer. The plate andnegative are then subjected to flood-exposure by UV light through thenegative. The areas exposed to the light cure, or harden, and theunexposed areas are removed (developed) to create the relief image onthe printing plate. Instead of a slip film, a matte layer may also beused to improve the ease of plate handling. The matte layer typicallycomprises fine particles (silica or similar) suspended in an aqueousbinder solution. The matte layer is coated onto the photopolymer layerand then allowed to air dry. A negative is then placed on the mattelayer for subsequent UV-flood exposure of the photocurable layer.

In a “digital” or “direct to plate” plate making process, a laser isguided by an image stored in an electronic data file, and is used tocreate an in situ negative in a digital (i.e., laser ablatable) maskinglayer, which is generally a slip film that has been modified to includea radiation opaque material. Portions of the laser ablatable layer areablated by exposing the masking layer to laser radiation at a selectedwavelength and power of the laser. Examples of laser ablatable layersare disclosed for example, in U.S. Pat. No. 5,925,500 to Yang, et al.,and U.S. Pat. Nos. 5,262,275 and 6,238,837 to Fan, the subject matter ofeach of which is herein incorporated by reference in its entirety.

After imaging, the photosensitive printing element is developed toremove the unpolymerized portions of the layer of photocurable materialand reveal the crosslinked relief image in the cured photosensitiveprinting element. Typical methods of development include washing theprinting element with various solvents or water, often with a brush.Other possibilities for development include the use of an air knife orheat plus a blotter. The resulting surface has a relief pattern thatreproduces the image to be printed and which typically includes bothsolid areas and patterned areas comprising a plurality of reliefprinting dots. After the relief image is developed, the printing elementmay be mounted on a press and printing commenced.

A “back exposure” step may also be performed prior to imaging thephotosensitive printing element (or immediately after imaging thephotosensitive printing element). “Back exposure” refers to a blanketexposure to actinic radiation of the photopolymerizable layer on theside opposite that which does, or ultimately will, bear the relief. Thisstep is typically accomplished through a transparent support layer andis used to create a shallow layer of photocured material, i.e., the“floor,” on the support side of the photocurable layer. The purpose ofthe floor is generally to sensitize the photocurable layer and toestablish the depth of relief.

The shape of the dots and the depth of the relief, among other factors,affect the quality of the printed image. It is very difficult to printsmall graphic elements such as fine dots, lines and even text usingflexographic printing plates while maintaining open reverse text andshadows. In the lightest areas of the image (commonly referred to ashighlights) the density of the image is represented by the total area ofdots in a halftone screen representation of a continuous tone image. ForAmplitude Modulated (AM) screening, this involves shrinking a pluralityof halftone dots located on a fixed periodic grid to a very small size,the density of the highlight being represented by the area of the dots.For Frequency Modulated (FM) screening, the size of the halftone dots isgenerally maintained at some fixed value, and the number of randomly orpseudo-randomly placed dots represent the density of the image. In bothinstances, it is necessary to print very small dot sizes to adequatelyrepresent the highlight areas.

Maintaining small dots on flexographic plates can be very difficult dueto the nature of the platemaking process. In digital platemakingprocesses that use a UV-opaque mask layer, the combination of the maskand UV exposure produces relief dots that have a generally conicalshape. The smallest of these dots are prone to being removed duringprocessing, which means no ink is transferred to these areas duringprinting (the dot is not “held” on plate and/or on press).Alternatively, if the dot survives processing they are susceptible todamage on press. For example small dots often fold over and/or partiallybreak off during printing causing either excess ink or no ink to betransferred.

Furthermore, photocurable resin compositions typically cure throughradical polymerization, upon exposure to actinic radiation. However, thecuring reaction can be inhibited by molecular oxygen, which is typicallydissolved in the resin compositions, because the oxygen functions as aradical scavenger. It is therefore desirable for the dissolved oxygen tobe removed from the resin composition, and/or to stop atmospheric oxygenfrom dissolving in the resin, before image-wise exposure so that thephotocurable resin composition can be more rapidly and uniformly cured.

The removal of dissolved oxygen may be accomplished in various ways. Forexample, the photosensitive resin plate may be placed in an atmosphereof inert gas, such as carbon dioxide gas or nitrogen gas, beforeexposure in order to displace the dissolved oxygen. Another approachinvolves subjecting the plates to a preliminary exposure (i.e., “bumpexposure”) of actinic radiation. During bump exposure, a low intensity“pre-exposure” dose of actinic radiation is used to sensitize the resinbefore the plate is subjected to the higher intensity main exposure doseof actinic radiation. The bump exposure is applied to the entire platearea and is a short, low dose exposure of the plate that reduces theconcentration of oxygen, which inhibits photopolymerization of the plate(or other printing element) and aids in preserving fine features (i.e.,highlight dots, fine lines, isolated dots, etc.) on the finished plate.Other efforts have involved special plate formulations alone or incombination with the bump exposure.

U.S. Pat. Pub. No. 2014/0141378 to Recchia, the subject matter of whichis herein incorporated by reference in its entirety, describes a methodof imaging a photocurable printing blank in a digital platemakingprocess that includes the steps of laminating an oxygen barrier membraneto a top of a laser ablated mask layer and exposing the printing blankto actinic radiation through the oxygen barrier membrane and mask layerto selectively crosslink and cure portions of the at least onephotocurable layer. The oxygen barrier membrane is removed prior to thedevelopment step. The presence of the oxygen barrier membrane producesprinting dots having desired characteristics. The method can also beused with an analog platemaking process that uses a negative instead ofan ablatable mask layer, or, in the alternative, the negative itself canbe used as the oxygen barrier layer.

U.S. Pat. Pub. No. 2014/005/7207 to Baldwin, the subject matter of whichis herein incorporated by reference in its entirety, describes the useof one or more UV LED assemblies in selectively crosslinking and curingsheet photopolymers can produce a relief image comprising flexo printingdots having desirable geometric characteristics.

As described in U.S. Pat. No. 8,158,331 to Recchia and U.S. Pat. Pub.No. 2011/0079158 to Recchia et al., the subject matter of each of whichis herein incorporated by reference in its entirety, it has been foundthat a particular set of geometric characteristics define a flexo dotshape that yields superior printing performance, including but notlimited to (1) planarity of the dot surface; (2) shoulder angle of thedot; (3) depth of relief between the dots; and (4) sharpness of the edgeat the point where the dot top transitions to the dot shoulder.

Flexo plates imaged by typical digital imaging processes tend to createdots with rounded tops. A rounded dot surface is not ideal from aprinting perspective because the size of the contact patch between theprint surface and the dot varies exponentially with impression force. Incontrast, a planar dot surface should have the same contact patch sizewithin a reasonable range of impression and is thus preferred,especially for dots in the highlight range (0-10% tone).

A second parameter is the angle of the dot shoulder. The shoulder anglecan vary depending on the size of the dots as well. There are twocompeting geometric constraints on shoulder angle—dot stability andimpression sensitivity. A large shoulder angle minimizes impressionsensitivity and gives the widest operating window on press, but at theexpense of dot stability and durability. In contrast, a lower shoulderangle improves dot stability but makes the dot more sensitive toimpression on press

A third parameter is plate relief, which is expressed as the distancebetween the floor of the plate and the top of a solid relief. The dotrelief is to a certain extent linked to the dot's shoulder angle.

A fourth characteristic is the presence of a well-defined boundarybetween the planar dot top and the shoulder. Dots made using standarddigital flexo photopolymer imaging processes tend to exhibit rounded dotedges. It is generally preferred that the dot edges be sharp anddefined. These well-defined dot edges better separate the “printing”portion from the “support” portion of the dot, allowing for a moreconsistent contact area between the dot and the substrate duringprinting. Edge sharpness can be defined as the ratio of r_(e), theradius of curvature (at the intersection of the shoulder and the top ofthe dot) to p, the width of the dot's top or printing surface, asdescribed for example in U.S. Pat. No. 8,158,331 to Recchia and U.S.Pat. Pub. No. 2011/0079158 to Recchia et al., the subject matter of eachof which is herein incorporated by reference in its entirety. For atruly round-tipped dot, it is difficult to define the exact printingsurface because there is not really an edge in the commonly understoodsense, and the ratio of r_(e):p can approach 50%. In contrast, asharp-edged dot would have a very small value of r_(e), and r_(e):pwould approach zero. In practice, an r_(e):p of less than 5% ispreferred, with an r_(e):p of less than 2% being most preferred.

In the pursuit of printing dots with improved shapes and improvedprinting performance, photopolymer printing plates have been introducedwith photoinitiators with particular chemical and performancecharacteristics. These new classes of photoinitiators, however, havecreated printing plates with increased sensitivity to normal ambientdaylight and normal indoor fluorescent lighting. These new printingplates have a tendency to cure relatively quickly in the presence ofnormal ambient daylight and/or indoor fluorescent lighting.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improvedphotocurable composition for producing flexographic relief imageprinting elements that is capable of producing printing dots havingdesired geometric characteristics.

It is another object of the present invention to provide a process ofmaking a relief image printing element having printing dots with desiredgeometric characteristics that does not require additional process stepsin the manufacturing process.

It is still another object of the present invention to provide a processof making a relief image printing element that does not require alteringof the type, power and incident angle of radiation during the exposurestep.

It is still another object of the present invention to provide a processof making a relief image printing element that can be conducted in thepresence of atmospheric oxygen while producing printing dots havingdesired geometric characteristics.

It is still another object of the present invention to provide animproved photosensitive printing plate formulation having improved cureefficiency.

It is still another object of the present invention to provide animproved photosensitive printing plate with improved stability in thepresence of normal ambient daylight and/or indoor fluorescent lighting.

To that end, in one embodiment, the present invention relates generallyto a photocurable composition for producing a relief image printingelement, the photocurable composition comprising:

-   -   a) an ethylenically unsaturated monomer;    -   b) a binder;    -   c) a photo initiator, the photoinitiator preferably exhibiting a        quantum yield of initiation (Qi) of more than 0.05 at a 365 nm        wavelength;    -   disposed on a substrate which has an optical density of from 0.5        to 5 in the    -   wavelength range of 365 nm to 450 nm, but also preferably allows        transmission of from 0.1% to 10% of incident light in the        wavelength range of 356 nm to 450 nm.

In another embodiment, the present invention relates generally to amethod of making a relief image printing element, the method comprisingthe step of:

-   -   a) providing at least one photocurable layer disposed on a        substrate, the at least one photocurable layer being capable of        being selectively crosslinked and cured upon exposure to actinic        radiation, the at least one photocurable layer comprising:        -   i) an ethylenically unsaturated monomer;        -   ii) a binder;        -   iii) a photoinitiator, the photoinitiator preferably            exhibiting a quantum yield of initiation (Qi) of more than            0.05 at a 365 nm wavelength;        -   disposed on a substrate which has an optical density of from            0.5 to 5 in the wavelength range of 365 nm to 450 nm but            also preferably allows transmission of from 0.1% to 10% of            incident light in the wavelength range of 365 nm to 45 nm;    -   b) imagewise exposing the at least one photocurable layer to        actinic radiation to selectively crosslink and cure portions of        the at least one photocurable layer; and    -   c) developing the relief image printing element to separate and        remove uncrosslinked and uncured portions of the at least one        photocurable layer to reveal the relief image therein;

wherein the relief image comprise a plurality of relief image printingdots, wherein the plurality of relief image printing dots exhibit anedge sharpness of the dots such that the ratio of the radius ofcurvature at the intersection of the shoulder and the top surface of thedot, r_(e), to the width of the top of the dot, p, is less than 5%.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 depicts printing dots produced in accordance with the presentinvention using different photoinitiators.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The inventors of the present invention have found that the use ofparticular photoinitiators in a photocurable printing plate compositionproduces print dots having desired geometric characteristics without theneed for additional process steps. Thus, the photocurable compositionsdescribed herein produce relief image printing plates having printingdots with the desired geometric characteristics without the need for abarrier layer. In addition, the process described herein can also beconducted in the presence of atmospheric oxygen. Lastly, the printingplates described herein have good stability prior to processing in thepresence of normal ambient daylight and indoor fluorescent lighting.

To that end, in one embodiment, the present invention relates generallyto a method of making a relief image printing element, the methodcomprising the step of:

-   -   a) providing at least one photo curable layer disposed on a        substrate, the at least one photocurable layer being capable of        being selectively crosslinked and cured upon exposure to actinic        radiation, the at least one photocurable layer comprising:        -   i) an ethylenically unsaturated monomer;        -   ii) a binder;        -   iii) a photoinitiator, the photoinitiator preferably            exhibiting a quantum yield of initiation (Qi) of more than            0.05 at a 365 nm wavelength;        -   disposed on a substrate which has an optical density of from            0.5 to 5 in the wavelength range of 365 nm to 450 nm but            also preferably allows transmission from 0.1% to 10% of            incident light in the wavelength range of 365 nm to 450 nm.    -   b) imagewise exposing the at least one photocurable layer to        actinic radiation to selectively crosslink and cure portions of        the at least one photocurable layer; and    -   c) developing the relief image printing element to separate and        remove uncrosslinked and uncured portions of the at least one        photocurable layer to reveal the relief image therein;

wherein the relief image comprise a plurality of relief image printingdots, wherein the plurality of relief image printing dots exhibit anedge sharpness of the dots such that the ratio of the radius ofcurvature at the intersection of the shoulder and the top surface of thedot, r_(e), to the width of the top of the dot, p, is less than 5%.

The present invention also relates generally to a photo curablecomposition for producing a relief image printing element, thephotocurable composition comprising:

-   -   a) an ethylenically unsaturated monomer;    -   b) a binder;    -   c) a photoinitiator, the photoinitiator preferably exhibiting a        quantum yield of initiation (Qi) of more than 0.05 at a 365 nm        wavelength;    -   disposed on a substrate which has an optical density of 0.5 to 5        in the    -   wavelength range of 365 nm to 450 nm but also preferably allows        transmission of from 0.1% to 10% of incident light in the        wavelength range of 365 nm to 450 nm.

The inventors of the present invention have found that the inclusion ofparticular photoinitiators into the photocurable composition having ahigher quantum yield of initiation produces a printing element withfiner and sharper printing dots. In one embodiment, thesephotoinitiators may comprise certain α-aminoketones.

The initiation rate of polymerization (Ri) was measured to evaluate thesuitability of various photoinitiators, which can be done by real-timeFTIR or RTIR.

Ri is described by Equation 1:Ri=I _(a) ·Q _(i)  (1)

I_(a) is the absorbed intensity (mW) and is calculated as set forthbelow in Equation 2.

Q_(i) is the quantum yield of initiation and is defined as the number ofinitiated polymerizing chains per absorbed photon. Q_(i) is influencedby all the photochemical/physical phenomena that can affect an excitedmolecule after absorption of one photon.I _(a) =I ₀·(1−10^(−OD))  (2)OD=ε·[PI]·L  (3)Wherein:

I₀=Incident intensity (mW)

ε=Extinction coefficient

[PI]=Photoinitiator concentration (mol/l)

L=Thickness (cm)

Q_(i) is calculated via an experimental determination of the rate ofpolymerization (R_(p)) and by the use of the propagation and terminationconstants (k_(p) and k_(t)) for acrylate monomers that are found in theliterature.

In order for a photoinitiator to react effectively, it must firsteffectively absorb the service wavelength, which means a high I_(a), andthus a high s value. Then, the absorbed energy must be converted in ahigh number of initiating radicals, which results in a high Q_(i) ratio.

In order to compare various photoinitiators, ε and Q_(i) were determinedfor three photoinitiators at 365 nm and the results are depicted inTable 1.

Photoinitiator ε_(365nm) (1 · cm⁻¹ · mol⁻¹) Q_(i-365 nm)2,2-dimethoxy-2-phenylacetophenone 141 0.0141-butanone-2-(dimethylamino)-2-[(4- 1247 0.081methylphenyl)methyl]-1-[4-(4- morpholinyl)phenyl] Diphenyl(2,4,6- 5180.118 trimethylbenzoyl)phosphine oxide

Printing plate formulations were prepared using the photoinitiatorsdescribed in Table 1 at the concentrations set forth in Table 2. Table 2also lists a range of concentration values that may be used for eachingredient of the sample photocurable composition.

Once the photocurable compositions were prepared using thephotoinitiators described above, the photocurable compositions wereimagewise exposed to actinic radiation and then developed using solventdevelopment to remove uncured photopolymer.

TABLE 2 Sample Photocurable Composition Example 1 Range (Wt. %) (Wt. %)Kraton ® D1114 (Rubber) 67.0 60-80 PB B-1000 13.0 10-20 HDDA 15.0 10-20BHT 1.92 0.5-5.0 Tinuvin 1130 0.075 0.02-0.20 Dye 0.01 0.005-0.05 Photoinitiator 3.0 1.5-5.0

Based on the results, it was determined that a Quantum yield ofinitiation (Qi) higher than about 0.05 at the 365 nm wavelength, morepreferably higher than about 0.075 at the 365 nm wavelength, and mostpreferably higher than about 0.08 at the 365 nm wavelength was capableof producing a printing plate having printing dots with the desiredgeometric characteristics as illustrated in FIG. 1.

A high extinction coefficient is also necessary but is not sufficient inand of itself for good initiation. Indeed, after the light absorption,the photoinitiator is promoted to its singlet then triplet states fromwhich it can undergo different reactions, including the generation ofradicals, quenching by the monomer, oxygen inhibition and thermaldeactivation. At this stage, there is already a risk that theeffectiveness of the photoinitiator is reduced, even for a highextinction coefficient molecule. Assuming that everything goes well andthe radicals production is dominant, the type of radicals produced maystill have different sensitivities towards oxygen depending on theirreactivities. Again, a high coefficient of extinction would notnecessarily be enough if these radicals have a long enough lifetime,making them too sensitive to oxygen and thus reducing theireffectiveness in initiating the crosslinking reaction.

Thus, it is desirable that the extinction coefficient be higher thanabout 300 1·cm⁻¹·mol⁻¹ at the 365 nm wavelength, more preferably higherthan about 400 1·cm⁻¹·mol⁻¹ at the 365 nm wavelength, and mostpreferably higher than about 500 1·cm⁻¹·mol⁻¹ at the 365 nm wavelength.

Based on the values of Qi and ε shown in Table 1, both1-butanone-2-(dimethylamino)-2-[(4-methylphenyl)methyl]-1-[4-(4-morpholinyl)phenyl]and Diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide are fasterphotoinitiators than 2,2-dimethoxy-2-phenylacetophenone. This accountsfor the smaller and sharper dots that were obtained using these productsas shown in FIG. 1. In addition, althoughDiphenyl(2,4,6-trimethylbenzoyl)phosphine oxide has a slightly larger Qithan1-butanone-2-(dimethylamino)-2-[(4-methylphenyl)methyl]-1-[4-(4-morpholinyl)phenyl],the much higher absorptivity of the latter allowed it to offset thisdifference and yield sharper dots. Thus, even though both1-butanone-2-(dimethylamino)-2-[(4-methylphenyl)methyl]-1-[4-(4-morpholinyl)phenyl]and Diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide yield good results,1-butanone-2-(dimethylamino)-2-[(4-methylphenyl)methyl]-1-[4-(4-morpholinyl)phenyl]seems to be faster than Diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide.

As can be seen from FIG. 1,1-butanone-2-(dimethylamino)-2-[(4-methylphenyl)methyl]-1-[4-(4-morpholinyl)phenyl]and Diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide yield flat and finedots because of their high initiation rate due to their highabsorptivity and quantum yield of initiation. It is expected that asimilar behavior would also result from other photoinitiators thatexhibit comparable properties.

In addition, one or more antioxidants such as1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene,butylated hydroxytoluene (BHT), alkylated phenols, e.g.,2-6-di-tert-butyl-4-methylphenol; alkylated bis-phenols, e.g.,2,2-methylene-bis-(4-methyl-6-tert-butylphenol);2-(4-hydroxy-3,5-di-tert-butylanilino)-4,6-bis-(n-octylthio)-1,3,5-triazine; polymerized trimethyldihydroquinone; and dilaurylthiopropionate can also be used in the compositions of the invention incombination with the above referenced additives to further tailor dotshapes in terms of dot angle, dot tops, etc. In one preferredembodiment, the antioxidant is1,3,5-trimethyl-2,4,6-tris-(3,5-di-tert-butyl-4-hydroxybenzyl)benzene,available from Albemarle under the tradename Ethanox 330.

The photocurable composition of the present invention comprises one ormore binders, monomers and plasticizers in combination with the one ormore photo-initiators described above.

The binder type is not critical to the photopolymer composition andmost, if not all, styrenic copolymer rubbers are usable in thecompositions of the invention. Suitable binders can include natural orsynthetic polymers of conjugated diolefin hydrocarbons, including1,2-polybutadiene, 1,4-polybutadiene, butadiene/acrylonitrile,butadiene/styrene, thermoplastic-elastomeric block copolymers e.g.,styrene-butadiene-styrene block copolymer, styrene-isoprene-styreneblock copolymer, etc., and copolymers of the binders. It is generallypreferred that the binder be present in at least an amount of 60% byweight of the photosensitive layer. The term binder, as used herein,also encompasses core shell microgels or blends of microgels andpre-formed macromolecular polymers.

Non-limiting examples of binders that are usable in the compositions ofthe instant invention include styrene isoprene styrene (SIS), acommercial product of which is available from Kraton Polymers, LLC underthe tradename Kraton® D1161; styrene isoprene butadiene styrene (SIBS),a commercial product of which is available from Kraton Polymers, LLCunder the tradename Kraton® D1171; styrene butadiene styrene (SBS), acommercial product of which is available from Kraton Polymers, LLC underthe tradename Kraton® DX405; and triblock copolymers based on styreneand isoprene, a commercial product of which is available from KratonPolymers, LLC under the tradename Kraton® D1114.

Monomers suitable for use in the present invention areaddition-polymerizable ethylenically unsaturated compounds. Thephotocurable composition may contain a single monomer or a mixture ofmonomers which form compatible mixtures with the binder(s) to produceclear (i.e., non-cloudy) photosensitive layers. The monomers aretypically reactive monomers especially acrylates and methacrylates. Suchreactive monomers include, but are not limited to, trimethylolpropanetriacrylate, hexanediol diacrylate, 1,3-butylene glycol diacrylate,diethylene glycol diacrylate, 1,6-hexanediol diacrylate, neopentylglycol diacrylate, polyethylene glycol-200 diacrylate, tetraethyleneglycol diacrylate, triethylene glycol diacrylate, pentaerythritoltetraacrylate, tripropylene glycol diacrylate, ethoxylated bisphenol-Adiacrylate, trimethylolpropane triacrylate, di-imethylolpropanetetraacrylate, triacrylate of tris(hydroxyethyl)isocyanurate,dipentaerythritol hydroxypentaacrylate, pentaerythritol triacrylate,ethoxylated trimethylolpropane triacrylate, triethylene glycoldimethacrylate, ethylene glycol dimethacrylate, tetraethylene glycoldimethacrylate, polyethylene glycol-200 dimethacrylate, 1,6-hexanedioldimethacrylate, neopentyl glycol dimethacrylate, polyethylene glycol-600dimethacrylate, 1,3-butylene glycol dimethacrylate, ethoxylatedbisphenol-A dimethacrylate, trimethylolpropane trimethacrylate,diethylene glycol dimethacrylate. 1,4-butanediol diacrylate, diethyleneglycol dimethacrylate, pentaerythritol tetramethacrylate, glycerindimethacrylate, trimethylolpropane dimethacrylate, pentaerythritoltrimethacrylate, pentaerythritol dimethacrylate, pentaerythritoldiacrylate, urethanemethacrylate or acrylate oligomers and the likewhich can be added to the photopolymerizable composition to modify thecured product. Monoacrylates including, for example, cyclohexylacrylate, isobornyl acrylate, lauryl acrylate and tetrahydrofurfurylacrylate and the corresponding methacrylates are also usable in thepractice of the invention. Especially preferred acrylate monomersinclude hexanediol diacrylate (HDDA) and trimethylolpropane triacrylate(TMPTA). Especially preferred methacrylate monomers include hexanedioldimethacrylate (HDDMA) and triemethylolpropane trimethacrylate (TMPTA).It is generally preferred that the one or more monomers be present in atleast an amount of 5% by weight of the photosensitive layer.

The photocurable layer also preferably contains a compatibleplasticizer, which serves to lower the glass transition temperature ofthe binder and facilitate selective development. Suitable plasticizersinclude, but are not limited to, dialkyl phthalates, alkyl phosphates,polyethylene glycol, polyethylene glycol esters, polyethylene glycolethers, polybutadiene, polybutadiene styrene copolymers, hydrogenated,heavy naphthenic oils, hydrogenated, heavy paraffinic oils, andpolyisoprenes. Other useful plasticizers include oleic acid, lauricacid, etc. The plasticizer is generally present in an amount of at least10% by weight, based on weight of total solids of the photopolymercomposition. Commercially available plasticizers for use in compositionsof the invention include 1,2-polybutadiene, available from Nippon SodaCo. under the tradename Nisso PB B-1000; Ricon 183, which is apolybutadiene styrene copolymer, available from Cray Valley; Nyflex222B, which is a hydrogenated heavy naphthenic oil, available from NynasAB; ParaLux 2401, which is a hydrogenated heavy paraffinic oil,available from Chevron U.S.A., Inc.; and Isolene 40-S, which is apolyisoprene available from Royal Elastomers.

Various dyes and/or colorants may also optionally be used in thepractice of the invention although the inclusion of a dye and/orcolorant is not necessary to attain the benefits of the presentinvention. Suitable colorants are designated “window dyes” which do notabsorb actinic radiation in the region of the spectrum that theinitiator present in the composition is activatable. The colorantsinclude, for example, CI 109 Red dye, Methylene Violet (CI Basic Violet5), “Luxol.” Fast Blue MBSN (CI Solvent Blue 38), “Pontacyl” Wool BlueBL (CI Acid Blue 59 or CI 50315), “Pontacyl” Wool Blue GL (CI Acid Blue102 or CI 50320), Victoria Pure Blue BO (CI Basic Blue 7 or CI 42595),Rhodamine 3 GO (CI Basic Red 4), Rhodamine 6 GDN (CI Basic Red I or CI45160), 1,1′-diethyl-2,2′-cyanine iodide, Fuchsine dye (CI 42510),Calcocid Green S (CI 44090) and Anthraquinone Blue 2 GA (CI Acid Blue58), etc. The dyes and/or colorants must not interfere with theimagewise exposure.

Other additives including antiozonants, fillers or reinforcing agents,thermal polymerization inhibitors, UV absorbers, etc. may also beincluded in the photopolymerizable composition, depending on the finalproperties desired. Such additives are generally well known in the art.

Suitable fillers and/or reinforcing agents include immiscible, polymericor nonpolymeric organic or inorganic fillers or reinforcing agents whichare essentially transparent at the wavelengths used for exposure of thephotopolymer material and which do not scatter actinic radiation, e.g.,polystyrene, the organophilic silicas, bentonites, silica, powderedglass, colloidal carbon, as well as various types of dyes and pigments.Such materials are used in amounts varying with the desired propertiesof the elastomeric compositions. The fillers are useful in improving thestrength of the elastomeric layer, reducing tack and, in addition, ascoloring agents.

Thermal polymerization inhibitors include, for example, p-methoxyphenol,hydroquinone, and alkyl and aryl-substituted hydroquinones and quinones,tert-butyl catechol, pyrogallol, copper resinate, naphthalamines,beta-naphthol, cuprous chloride, 2,6-di-tert-butyl-p-cresol, butylatedhydroxytoluene (BHT), oxalic acid, phenothiazine, pyridine, nitrobenzeneand dinitrobenzene, p-toluquinone and chloranil. Other similarpolymerization inhibitors would also be usable in the practice of theinvention.

Using the photoinitiators described herein, it is possible to produceprinting plates having printing dots that exhibit desired geometriccharacteristics for printing, including planarity of a top surface ofthe dots and edge sharpness of the dots. Furthermore, these desiredcharacteristics can be achieved without using an oxygen barrier layer inthe process and without altering the type, power or incident angle ofradiation during the exposure step. Finally, the method described hereinmay also be conducted in the presence of atmospheric oxygen, meaningnormal atmospheric air and normal atmospheric oxygen concentrations atthe location where the process is being conducted. Thus no specialcontrol of the gaseous content of the atmosphere in which the process isconducted is required.

The use of the photoinitiators with the characteristics described hereinsurprisingly results in printing elements that are significantly moresensitive to normal ambient daylight and/or indoor fluorescent lightingthan typical printing elements. This sensitivity results in shortershelf life, unpredictable storage stability and the need to morecarefully handle the printing elements during processing. The inventorshave found that the storage stability of these printing elements can beincreased substantially by employing a substrate that has an opticaldensity of from 0.5 to 5, more preferably from 1 to 5, in the wavelengthrange from 365 nm to 450 nm but also preferably allows transmission offrom 0.1% to 10% of incident light in the wavelength range of 365 to450. This also increases the shelf life of the printing elements andallows for easier handling in the presence of ambient light. This alsoallows for the establishment of a floor layer through exposure of theprinting element to actinic radiation through the substrate. Further,for the best storage stability, the inventors have also determined thatthe printing elements should be stored in their boxes with the absorbingsubstrate faced up towards the opening of the box.

As noted, the photocurable composition described herein is disposed on asubstrate. In accordance with this invention, the substrate should havean optical density of from 0.5 to 5, more preferably from 1 to 5, in thewavelength range from 365 nm to 450 nm. The substrate layer should,however, preferably allow transmission of from 0.1% to 10% incidentlight in the wavelength range of 365 nm to 450 nm nm through thesubstrate into the photocurable layer composition. Optical density is astandard measure of the attenuation of light passing through asubstance. Optical density reports orders of magnitude of attenuation ofthe specified wavelength range of light. So for instance an opticaldensity of 1 means that the intensity (energy) of light in the selectedwavelength range was attenuated by a factor of 10 in passing through thesubstrate, and an optical density of 2 reports that the intensity(energy) of light in the selected wavelength range was attenuated by afactor of 10² in passing through the substrate. The substrate ispreferably a flexible polymeric sheet, and is most preferablypolyethylene terephthalate (PET). In order to achieve the requiredoptical density required by this invention, the flexible polymer or PETshould be doped with an absorber of UV light in the selected wavelengthrange of 365 nm to 450 nm. Ultraviolet absorbing compounds that could beused to dope the flexible polymer substrate to increase its opticaldensity include benzophenones, diethylamino hydroxybenzoyl hexylbenzoate, ethylhexyl triazone, oxybenzone, octinoxate, octocrylene,PABA, and sulisobenzone. Commercially UV absorbing compounds are sold bya variety of companies, including BASF under its tradename, Uvinul®.These UV absorbers are strongly absorbing, such that very lowconcentrations in the flexible polymer or PET will substantiallyincrease the optical density thereof.

What is claimed is:
 1. A method of making a relief image printingelement comprising a plurality of relief printing dots, the methodcomprising the step of: a) providing at least one photocurable layerdisposed on a substrate, the at least one photocurable layer beingcapable of being selectively crosslinked and cured upon exposure toactinic radiation, the at least one photocurable layer comprising: i) anethylenically unsaturated monomer; ii) a binder; and iii) aphotoinitiator, the photoinitiator exhibiting a quantum yield ofinitiation (Qi) of more than 0.05 at a 365 nm wavelength; wherein thesubstrate comprises a flexible polymer sheet with an ultravioletabsorbing compound incorporated therein, and wherein the substrate hasan optical density from 0.5 to 5 in the wavelength range of 365 nm to450 nm but allows transmission of from 0.1% to 10% of incident light inthe wavelength range of 365 nm to 450 nm; b) imagewise exposing the atleast one photocurable layer to actinic radiation to selectivelycrosslink and cure portions of the at least one photocurable layer; andc) developing the relief image printing element to separate and removeuncrosslinked and uncured portions of the at least one photocurablelayer to reveal a relief image therein, wherein the relief imagecomprises the plurality of relief printing dots, and wherein theplurality of relief printing dots exhibit an edge sharpness of the dotssuch that the ratio of the radius of curvature at the intersection ofthe shoulder and the top surface of the dot, r_(e), to the width of thetop of the dot, p, is less than 5%, and wherein the printing elementexhibits improved storage stability and shelf life.
 2. The methodaccording to claim 1, wherein the step of imagewise exposing the atleast one photocurable layer to actinic radiation is conducted in thepresence of normal atmospheric oxygen concentrations at the locationwhere the method is being conducted.
 3. The method according to claim 1,wherein the photoinitiator exhibits a quantum yield of initiation (Qi)greater than 0.075 at the 365 nm wavelength.
 4. The method according toclaim 3, wherein the photoinitiator exhibits a quantum yield ofinitiation (Qi) greater than 0.08 at the 365 nm wavelength.
 5. Themethod according to claim 1, wherein an extinction coefficient of thephotoinitiator is greater than 300 1·cm⁻¹·mol⁻¹ at a 365 nm wavelength.6. The method according to claim 5, wherein the extinction coefficientof the photoinitiator is greater than 400 1·cm⁻¹·mol⁻¹ at the 365 nmwavelength.
 7. The method according to claim 6, wherein the extinctioncoefficient of the photoinitiator is greater than 500 1·cm⁻¹·mol⁻¹ atthe 365 nm wavelength.
 8. The method according to claim 1, wherein thephotoinitiator is selected from the group consisting of1-butanone-2-(dimethylamino)-2-[(4-methylphenyl)methyl]-1-[4-(4-morpholinyl)phenyl],2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone-1,Diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide,phenylbis(2,4,6-trimethylbenzoyl) phosphine oxide, and combinations ofone or more of the foregoing.
 9. The method according to claim 1,wherein the photoinitiator is present in the at least one photocurablelayer at a concentration of between 1.5 and 5.0 percent by weight. 10.The method according to claim 9, wherein the photoinitiator is presentin the at least one photocurable layer at a concentration of between 2.0and 3.5 percent by weight.
 11. The method according to claim 1, whereinthe substrate has an optical density from 1 to
 5. 12. A printing elementcomprising: a) at least one photocurable layer disposed on a substrate,the at least one photocurable layer being capable of being selectivelycrosslinked and cured upon exposure to actinic radiation, the at leastone photocurable layer comprising: i) an ethylenically unsaturatedmonomer; ii) a binder; and iii) a photoinitiator, the photoinitiatorexhibiting a quantum yield of initiation (Qi) of more than 0.05 at a 365nm wavelength; wherein the substrate comprises a flexible polymer sheetwith an ultraviolet absorbing compound incorporated therein and whereinthe substrate has an optical density from 1 to 5 in the wavelength rangeof 365 nm to 450 nm but also allows transmission of 0.1% to 10% ofincident light in the wavelength range of 365 nm to 450 nm, and whereinthe printing element exhibits improved storage stability and shelf life.13. The printing element according to claim 12, wherein thephotoinitiator exhibits a quantum yield of initiation (Qi) greater than0.075 at the 365 nm wavelength.
 14. The printing element according toclaim 13, wherein the photoinitiator exhibits a quantum yield ofinitiation (Qi) greater than 0.08 at the 365 nm wavelength.
 15. Theprinting element according to claim 12, wherein an extinctioncoefficient of the photoinitiator is greater than 300 1·cm⁻¹·mol⁻¹ at a365 nm wavelength.
 16. The printing element according to claim 15,wherein the extinction coefficient of the photoinitiator is greater than400 1·cm⁻¹·mol⁻¹ at the 365 nm wavelength.
 17. The printing elementaccording to claim 16, wherein the extinction coefficient of thephotoinitiator is greater than 500 1·cm⁻¹·mol⁻¹ at the 365 nmwavelength.
 18. The printing element according to claim 12, wherein thephotoinitiator is selected from the group consisting of1-butanone-2-(dimethylamino)-2-[(4-methylphenyl)methyl]-1-[4-(4-morpholinyl)phenyl],2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone-1,Diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide,phenylbis(2,4,6-trimethylbenzoyl) phosphine oxide, and combinations ofone or more of the foregoing.
 19. The printing element according toclaim 12, wherein the photoinitiator is present in the at least onephotocurable layer at a concentration of between 1.5 and 5.0 percent byweight.
 20. The printing element according to claim 19, wherein thephotoinitiator is present in the at least one photocurable layer at aconcentration of between 2.0 and 3.5 percent by weight.